The rules state that you must move forward if possible. This takes a lot of strategy out of movement. But also raises a question. At what point during the turn around the canyon does "forward" change directions? Does it ever become diagonal? Or does it just shift to the side at some point? It would be nice to get over closer to the canyon leading up to the turn, but not possible if you are in the leading spot, because you must move "forward".

It's a poor translation, indeed. The German rules do not say that you must move the canoes forward where possible. They clearly state that you must move the canoes in an 'optimal' way. ("...Die Kanus müssen aber 'optimal' gesetzt werden...".)

However, the German rules do say that at the end of their movement the canoes must have been moved as far towards the finish as possible. Therefore, you will have to / may be able to move diagonally at certain times.